1. Field of the Invention
The present invention relates to a transmitter, and more particularly, to a transmitter having low power consumption.
2. Description of the Related Art
In the field of signal transmissions, in order to allow the signal to be successfully transferred from a transmitter to a receiver, the impedance matching should be considered when the transmitter and the receiver are being designed. In other words, the impedances of the transmitter and the receiver are designed to match the characteristic impedance of the transmission medium (ex: cable).
FIG. 1 shows a block diagram of a transmission system according to the prior art. As shown in FIG. 1, assume that the equivalent impedance of the transmission medium 110 is R. Therefore, the impedances of the transmitter 100 and the receiver 120, which correspond to the transmission medium side, are substantially equal to the impedance R of the transmission medium. This helps prevent the transmission signal from reflection, and optimizes the transmission efficiency (that is, the voltage level Va of the node A is substantially equal to the voltage level Vb of the node B).
As is known, a conventional transmitter can be divided into two types of transmitters, the current-mode transmitter and the voltage-mode transmitter. Please refer to FIG. 2, which is a diagram showing a current-mode transmitter 200 and a receiver 220. Here, assume that the impedance of the transmission medium 210 is R. Therefore, for the purpose of impedance matching, the equivalent input impedance Rb of the receiver 220 is equal to R. When the signal is being transferred, the voltage level Va of the node A is equal to the voltage level Vb of the node B. At this time, for the transmitter 200, the impedance of the input impedance Ra is also equal to R. Therefore, the current outputted from the transmitter 200 is 2Vb(t)/R. Furthermore, in order to make sure that the current can be definitely outputted, the working voltage Vdd of the transmitter 200 must be larger or equal to the maximum of Vb(t). Therefore, the power consumption of the entire transmitter 200 can be represented by the following equation (1):Power consumption≧Vb(t)max*2Vb(t)/R  equation (1)
Please refer to FIG. 3, which is a diagram showing a voltage-mode transmitter 300 and a receiver 320. Similarly, the equivalent input impedance Rb of the receiver 320 is equal to the impedance R of the transmission medium. The voltage level Va of the node A is equal to the voltage level Vb of the node B. At this time, for the transmitter 300, the impedance of the input impedance Ra is equal to R. Therefore, the current outputted form the transmitter is Vb(t)/R. For the node C, the voltage level Vc(t) of the node C is equal to 2Vb(t). Therefore, in order to make sure that the current of the transmitter 300 can be outputted. The working voltage Vdd of the transmitter 300 should be larger or equal to the maximum of 2Vb(t). And the power consumption of the transmitter 300 can be represented by the following equation (2):Power consumption≧2Vb(t)max*Vb(t)/R  equation (2)
Obviously, when the current-mode transmitter is utilized, the needed current is larger, but needed working voltage is lower. On the other hand, when the voltage-mode transmitter is utilized, the needed working voltage is larger, but the needed current is lower. Please refer to equations (1) and (2), it is easily seen that regardless that the of whether above-mentioned current-mode transmitter and voltage-mode transmitter are utilized, the lowest power consumptions of the current-mode transmitter and voltage-mode transmitter are both equal to 2Vb(t)max*Vb(t)/R, which needs to be reduced.